This invention relates to a connectionless communication network. A method is disclosed for transporting broadband services requiring Quality of Service (QoS) guarantees on a connectionless network and ensuring that the service maintains the Quality of Service determined at the start of a communication session throughout the duration of that session.
Connectionless network operation has been traditionally used in data communications. Its main features are not to require a connection setup phase and to route each packet of a connection independently. No connection related state is then needed in the switch and a topological change in a node (i.e., its failure or its hand-over) only necessitates the update of the routing table contained in its neighbours networks based on the Internet protocol (IP) are typically connectionless. Wireless and satellite data networks are often designed for connectionless operation in order to avoid the need for explicit connection hand-overs due to the physical mobility of users or systems. Hence, in a satellite network, connectionless operation removes the need for connection related states to be held in the sky segment. Each packet of information is self routed so the network is able to adapt to changes due to hand-overs between satellites. Each hand-over can be interpreted as a change of topology.
A method of operating an ATM network by using a software representation comprising a virtual network model is described in specification No. EP-A-748142. A description of ATM-based routing in LEO/MEO satellite networks is given by M Werner et al. in IEEE Journal on Selected areas in Communications, Vol. 15, No. 1, January 1997, pages 69-82.
Transport of broadband services requiring Quality of Service (QoS) guarantees (i.e., multimedia services) over a connectionless network is a major issue since traditionally this kind of network has only offered best-effort services. QoS guarantees are usually provided through connection orientation, as in the case of BISDN networks based on ATM. For such networks, a virtual circuit is established during a set-up phase and then routing of individual packets is simplified (i.e., no decision needs to be made) since the route (i.e., the virtual circuit) has already been chosen. In that case, the trade-off between connection orientation and connectionless operation is: the need for a call set-up phase; much less processing per packet; the necessity to keep connection related state in the switches involved in the connection; easier handling of QoS guarantees; no need for end to end re-sequencing; and the need to perform a new connection set-up phase for any topological change involving a node in the route. This last point is crucial for a low earth orbit (LEO) based satellite network where topological changes are very frequent.
Against this background the invention provides a connectionless digital communication network having changeable topology and means for maintaining the bandwidth available when the topology changes.
An object of the invention is to provide an improved connectionless digital communications network.
A further object of the invention is to provide an improved method of traffic routing in a communications network of varying topology.
According to a first aspect of the invention there is provided a method of determining routing of traffic in a communications network incorporating a plurality of non-geostationary satellite nodes and having a changing topology, the method comprising providing a virtual model of the network, said model consisting of fixed virtual nodes, determining within the model routing for the traffic in the real network, reflecting said topology changes in the network by temporarily associating the virtual nodes of the model with corresponding real nodes so as to provide each said real node with a respective virtual node identity, communicate the virtual identity of each said real node to its neighbouring nodes, and performing traffic routing in the real network with reference to the current embodiment of the virtual network.
According to another aspect of the invention there is provided a communications network incorporating a plurality of non-geostationary satellite nodes and having a changing topology, the network including a virtual model consisting of fixed virtual nodes, there being means for determining within the virtual model routing for traffic in the real network, means for reflecting said topology changes in the network by temporarily associating the virtual nodes of the model with corresponding real nodes so as to provide each said real node with a respective virtual node identity, and means for communicate the virtual identity of each said real node to its neighbouring nodes so as to perform traffic routing in the real network QoS guarantees can be achieved without requiring a connection oriented function within the network so that the network endpoints remain insensitive to topological change within the network.
In a preferred embodiment, a fixed virtual model of the network is maintained and in a physical nodes advertises to adjacent physical nodes with which it has direct links, which virtual node it embodies.
Preferably before a message is transmitted from one user to another across the network, a path request is sent to a connection control centre, checks are made that a user virtual node for said one user and the virtual nodes adjacent that user virtual node all have sufficient available time slot capacity. If the adjacent virtual nodes all concur then the slot can be sustained by all of the satellites which are required to receive hand-over of the corresponding user cell.
The network preferably includes topology groups of virtual nodes, being fully interconnected within the topology group within the model.
Services are preferably ATM based.
In one implementation, the network nodes include satellites with onboard switching functions.
In another implementation the network is ground based.
A connection control system preferably directs connectionless packets to follow paths selected from a virtual model of the network.
In one preferred alternative, a user network interface is part of the plesiochronous digital hierarchy.
In another alternative, a user network interface is part of the synchronous digital hierarchy.
In yet another alternative, a user network interface is based on the asynchronous transmission mode.
The invention extends to a connection control system for a connectionless communication network, said control system acting to direct connectionless packets to follow paths selected from a virtual model of the network.
In a preferred embodiment of the invention, the physical network consists of moving nodes. A virtual model of the network, consisting of fixed virtual nodes, is maintained in a connection control system. This virtual model is used to control the route that packets will take through the real network. As topology changes happen in the network then the virtual nodes of the model become embodied by different real nodes and communicate their virtual identity to connected nodes. Routing of the packets is performed with reference to the virtual network so that packet routing follows the current embodiment of the virtual network. By ensuring that the topology changes re-establish the bandwidth available within the virtual model then the system guarantees that the quality of service established for the session is maintained.